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Session 47 - Circumstellar Disks & Shells.
Display session, Thursday, January 08
We report the results of a study of spectropolarimetric data for a B[e] star, HD 50138, and a Herbig Ae/Be star, AB Aur, obtained contemporaneously with the WUPPE on Astro-2 and at the Pine-Bluff Observatory in March 1995. The data cover a spectral region from 1450 to 10500 ÅP>A position angle (PA) flip by 70 degrees occurring in the UV is detected for AB Aur. This confirms previous indirect suggestions about the non-spherical shape of its circumstellar envelope, and implies a rather small density contrast between the equatorial and polar regions. A new, more accurate, estimate of the interstellar component of the object's polarization is made using the data on nearby stars with parallaxes measured by HIPPARCOS. The wavelength dependence and PA of the intrinsic polarization imply that the orientation of the circumstellar disk plane is close to that of the galactic plane, and inclined at a small angle with respect to the line of sight.
No PA flip was found for HD 50138. We argue that polarization in both the ultraviolet and optical regions is due to electron scattering in a geometrically thin equatorial disk, like in classical Be stars. The almost flat wavelength dependence of intrinsic polarization suggests that it is weakly attenuated by circumstellar dust. This implies a certain inclination angle between the disk plane and the line of sight which may be smaller than for AB Aur because the opening angle of the disk may be smaller. The PA of 160 degrees found for HD 50138 is the same as that of the UV polarization of HD 45677, a similar emission-line star displaying a PA flip. Since the UV polarization of the latter star is produced in the polar regions of its envelope, the plane of the disk in HD 50138 appears to be perpendicular to that in HD 45677 and to the galactic plane.
This work has been supported under NASA contract NAS5-26777 with the University of Wisconsin.
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